Because of the advantageous electronic properties exhibited by various materials at cryogenic temperatures, various machines have been developed for cooling electronic devices so that they may be operated at cryogenic temperatures. Many such refrigeration systems have used Stirling cycle cryogenic coolers. Such existing machines, however, are relatively large, bulky, inefficient and noisy machines generating substantial vibration. While technology has developed to permit a remarkable miniaturization of electronic circuitry, thereby permitting large numbers of electronic circuits to be contained within a relatively small volume, the apparatus which is available for cooling such circuits is relatively large and consequently adds a substantial, additional volume and weight to cryogenic electronic circuitry. There is, therefore, a need for an efficient Stirling cycle cryocooler which can be miniaturized so that its size and weight are compatible with and do not add substantially to that of the electronic circuitry but are, nonetheless, capable of pumping heat at a sufficient rate to maintain the cryogenic temperatures.
A measure of the size and effectiveness of a cooler in pumping thermal energy is its specific capacity. Specific capacity is the ratio of the quantity of thermal energy which the machine can pump to a quantitative measure of its size or weight. Thus, a cryocooler must not only be able to pump sufficient thermal energy from the electronic device to maintain it at a cryogenic temperature, but should do so with the smallest possible size or weight. Consequently, the higher the specific capacity, the more desirable is the cooler.
The prior art has recognized that the specific capacity of a Stirling cycle cooler can be increased and therefore improved by operating the cooler at a higher frequency. A sufficiently high, thermal energy pumping rate can be maintained if the cooler is made smaller, but the frequency of its operation is increased so that more thermal energy pumping cycles occur each second.
However, the prior art has also recognized that entropy generating processes (ie, irreversibilities) have imposed an upper limit on the frequency of operation of Stirling cycle machines. As the operating frequency is increased, viscous dissipation resulting from the friction of the working fluid with the internal passage walls of the Stirling cryocooler also increases. As a result, more work is required to move the gas back and forth through the passages of the Stirling cycle machine at higher frequencies. In addition, the apparent thermal conductivity of the working fluid in the regenerator increases causing a larger amount of heat to be conducted into the cold end of the machine, and heat transfer in heat exchangers is reduced. The latter effects occur as a result of certain phase relationships between the periodic variations in axial mass flow and radial temperature gradient which arise in oscillatory flow. As a result, the amount of heat that must be pumped by the cryocooler is increased while the effectiveness of the cryocooler in exchanging heat with its surroundings is reduced. Consequently, increasing the frequency reduces the heat lifted. Therefore, the prior art has come to accept an upper frequency limit for Stirling cycle machines on the order of 50 Hz and a machine constructed to operate at 120 Hz is believed to be the highest frequency Stirling machine ever built.
There is, therefore, a need for a Stirling cryocooler which can lift heat at a sufficient rate to maintain electronic devices at cryogenic temperatures, but which also has a sufficiently small size and weight that it exhibits a specific capacity which is acceptable and compatible with the equipment which utilizes these electronic circuits.
Recent years have also seen the development of a micromachine technology. While such machines utilize mechanical devices, such as motion conversion linkages, mechanical advantage mechanisms, power trains, valves, diaphragms, cantilever beams and the like, which have configurations and modes of operation like conventional mechanical devices, they have a size on the order of a few millimeters or smaller.
Although Stirling cycle engines have long been used as mechanical prime movers, there is a need for a microminiature Stirling cycle engine for use with developing micromachine technology.